Control system



Oct. 20, 1942- E. D. SCHNEIDER CONTROL SYSTEM Filed March l, 1941 5 Sheets-Sheet l inventor: Elbert D. Schneider; be, //wy/jw is Attormeg.

Oct. 20, 1 942. E. D. SCHNEIDER 2,299,501

CONTROL SYSTEM Filed MaIGh l, 1941 3 Sheets-Sheet 2 rw; /48 Inventor;

`|ber`t D. Schneider,

His AJGOP hey.

Oct. 20, 1942. E, D. SCHNEIDER CONTROL SYSTEM 5 Sheets-Sheet 3 Filed March l, 1941 lrvemter Elbert 'D Schneider,

His Attorney.

Patented oef. 2o, 1942 CONTROL SYSTEM Elbert D. Schneider, Scotia, N. Y., assignor to General Electric Company, a corporation of New York Application March 1, 1941, Serial No. 381,286

21 Claims.

My invention relates\ to control systems, and although not limited thereto, it has application to electric control systems for furnaces.

Furnaces which burn a fluid fuel, such as oil orgas, usually employ an electric control system which includes a thermostat in the space to be heated so that when there is a call for heat-"a `fuel pump motor and electric ignition arrangement are'energized so that the fuel may be pumped to the furnace burner an'd ignited. If the fuel fails to ignite within a predetermined interval of time after the fuel pump has been operated, or should th'e flame during the operation of the furnace for any reason become extinguished, it is desirable that the burner of the furnace be shut down independently of the thermostat so as to prevent a quantity of unburned fuel from being pumped into the furnace. Heretofore, flame detector control arrangements have been provided with a heat responsive device within the furnace which device after a predetermined time delay provided by some mechanical timing arrangement, will turn oli the burner if the device does not become heated by the iame. rangement employing the rectifier characteristics of the flame have been employed, as described in Patent 2,112,736--Cockre1L which is assigned to the same assignee as this present invention, and

in order to obtain a suitable time delay in which the fuel is given time to ignite, a mechanical timing arrangement has been employed, as described in Patent 2,l36,256-Sweet, which is assigned to the same assignee as this present invention.

Furthermore, the general idea of providing a combined time delay and llame detecting furnace control system by the employment of an electric discharge device is disclosed in the Crago Patent 2,243,071, granted May 27, 1941, and which is assigned to the same assignee as this present invention. Since such a combined time delay and flame detecting furnace control system must provide for automatic response of the electric discharge device at any time tothe establishment and disestablishment of a main control condition, i. e., normal room temperature, and also provide for automaticresponse of the electric discharge device at any time to the establishment and disestablishment of a second normal control condition, i. e., proper ignition of the fuel in the furnace, While concurrently employing the electric discharge device for automatically limiting the interval of time permitted Also, a. flame control arfor operation of the fuel supply device in re- 55 spense to the rst control condition without successful establishment of the second control condition, some difficulties in obtaining the desired time delay control may be encountered. The control of the time delay action of the electric discharge device is complicated by the fact that the initiation of operation of the fuel supply device may occur at any time under automatic control and may follow certain extraneous and uncontrollable conditions, such for example as accidental failure of the electric power supply for the electric discharge device, accidental failure of combustion in the furnace after its initial successful establishment or extraneous changes in the main control condition such as might develop on opening doors or windows of the enclosure being heated, or even raising or lowering the desired room temperature to be held by manual adjustment of the Calibrating mechanism of the room thermostat.

Further timing control complications are encountered where the termination of the time delay action of the electric discharge device is effected by the detection of the establishment of the second condition in response to variation of an electrical characteristic of the ignited fuel, such for example, as the rectier characteristic of the flame. Such rectifier characteristic of the flame may be initially somewhat unstable since the flame may be initially somewhat unstable, and where the time delay action of the electric discharge device is made dependent upon a rectifier characteristic thereof, which may at times become relatively feeble and unstable, the time delay control dimculties are multiplied, particularly where both the rectifier characteristic of the llame and the rectifier characteristic of the electric discharge device operate reversely upon the control electrode of the electric discharge device,

Since the time delay control of the electric discharge device is quite critical for safe operation of the. automatic furnace control system under all of the varied normal and abnormal control conditions just noted, improvements in the reliability and consistency of th'e electric discharge time delay control are desirable.

It is, therefore, van object of my inventionto provide a control system for an electrically operated device with an improved electric discharge timing control arrangement for providing a predetermined time between the initiation of operation of the electrically operated device and a time in which the electrically operated device will be shut down if it has not at that time attained a predetermined normal condition of operation.

Another object of my invention is to provide an improved electric discharge timing control system for allowing a furnace a predetermined time in winch to attain a normal operating condition, and if such condition is not attained within that predetermined time, the furnace will be deenergized, independently of the thermostat in the space to be heated.

A further object of my invention is to provide an improved electric-discharge timing control arrangement for a domestic furnace employing a fluid fuel, which is simple in construction, and reliable in operation.

Further objects and advantages of my invention will become apparent from the following description referring to the accompanying drawings, and the features of novelty which characterize my invention will be pointed out with particularity in the claims annexed to and forming a part of this specification. Y

In the drawings, Fig 1 illustrates diagrammatically a, furnace control system which is provided with' an embodiment of my invention; and Figs. 2, 3, 4 and 5 illustrate modifications of the control arrangement, illustrated in Fig. 1.

In the illustrated embodiments of my invention, I have provided a control system for deenergizing a burner of a furnace if a normal flame condition does not obtain Within a predetermined time after the burner has been energized. This time delay is provided through a control circuit including an electric discharge device, which, upon a call for heat by a thermostat within the room to be heated, energizes the burner and ignition equipment for the furnace through suitable relay means after a predetermined normal current is flowing in the anodecathode circuit. Upon actuation of the burner a suitable circuit is rendered operative so that a unidirectional biasing voltage is built up at a control electrode of the tube at a predetermined rate, vso that when the control electrode becomes sufficiently negative with respect to the cathode, the plate current will have attained a suincient minimum value so that the burner mechanism will be shut down if a normal flame condition does not obtain within that time. In order to obtain the proper unidirectional grid bias in a suitable time, I may employ any one of several circuits which will be described in detail as the description proceeds.

`Referring more particularly to Fig. l of the drawings, I have illustrated a control system for a domestic furnace which burns a fluid fuel, such as oil, the combustibles being provided at the furnace through a burner arrangement indicated generally by the numeral I0. This burner arrangement is of the conventional type and includes an electric motor for operating a compressor and pump and an ignition transformer which is connected to energize, electrodes I2 and I3, the latter being placed adjacent the nozzle of the burner arrangement in order to ignite the mixture of fuel and air as it passes therethrough. Instead of a burner arrangement for a domestic furnace, any other suitable electrically operated device, in which a time delay is desired between the time the operation of the device is initiated until it is desirable to deenergize the device if a predetermined normal condition of operation thereof has not been attained, may be controlled with my electric control system. The energization of the burner mo- 2| back to the line I1. A heat responsive device or thermostat 22 is provided in the space to be heated for initiating operation of my control circuit when heat is desired. Any other suitable condition responsive device may be employed foi` controlling a suitable electrically operated device I0. A primary winding 23 of a suitable transformer 24 is adapted to be connected acrossA lines I6 and |1 through conductors 25, 2B, and 21 when the thermostat 22 is actuated. 'Ihe transformer 24 is provided with a secondary winding 28 for energizing a suitable electric discharge device 29. This electric discharge device includes the usual anode 30 and cathode 3|, and an arrangement for suitably heating the cathode is provided through the heater 32 which is connected across a secondary winding 33 of the transformer 24. The cathode 3| is connected t0 one side of the secondary winding 28 through the conductor 34 and the other side of the secondary winding 28 is connected to the anode of the discharge device through a conductor 35, operating coil 36 of the relay l5, and a conductor 31. Thus, it will be seen that when a predetermined normal current is flowing between the anode and cathode, which is sufficient to pick up the relay I5, the contacts I4 will be closed in order to start the burner motor of the furnace and energize its ignition transformer Since the electric discharge device is a half- Wave unidirectional rectifier, a suitable capacitor 38 may be connected in shunt with the operating coil 36 in order that it will not vibrate. In order to be able to control the discharge device a control member, such as a control electrode 39 is provided. A rectifying plate 48 is also provided which is suitably connected to a control electrode biasing circuit so that when the burner I0 is energized suitable unidirectional biasing voltage will be produced on the control elecl trode 39 at a predetermined rate, and when the electrode 39 has been made sufficiently negative with respect to the cathode 3| the anode-cathode current will have decreased to a predetermined value so that the relay I5 will drop out and deenergize the burner mechanism IB. The control electrode biasing circuit includes a secondary winding 4| of the transformer 24 and a suitable electric storage device, such as a capacitor 42. One side of the capacitor 42 is connected to the secondary winding 28-and to the cathode 3| through the conductor 3.4, and the other side of the capacitor 42 is connected to one end of the winding 4| through a resistor 43, the other end of the winding 4| being connected to the rectifying plate 40 through contacts 44 and a conductor 45. A resistor 46 is connected in shunt with the capacitor 42 and the terminal of the capacitor, which, as it will be seen becomes negative when it is charged, is connected to the control electrode 39 through a resistor' 41 and a conductor 48. Thus the control electrode biasing circuit is in effect-separate from the capacitor charging This prevents the alternating current circuit.

2,299,501 voltage produced in the winding 4I4 from being impressed upon the control electrode 39. VConsequently, the control electrode is subject only to the unidirectional biasing voltage produced by charging of the capacitor 42. The contacts 44 for closing the capacitor charging circuitlare controlled by a relay 49 which is in turn energized through contacts 50 which `close when the relay I5 picks up. Thus,A an operating coil 5I of the relay 49 is connected to. the line I1 through the conductor 21 and is connected to the other line I6 through a conductor 52, contacts 58, and a conductor 53. Contacts 54 may be provided on the relay 49 so as to provide a holding circuit for the relay 49 even though the relay I5 drops out. The contacts-54 are connected to the line I6 through the conductor 26 and the thermostat 22 and the conductor 25, and thecontacts are connected to the line I1 through coil 5| and conductors 55 and 21.

The operation of the control system as illustrated in Fig. 1 will now be described. When the thermostat 22 calls for heat it closes its contacts and energizes the transformer 24 so that a voltage is applied across the anode and cathode. Also, the secondary winding 3'3 is energized so that the cathode is heated up and the plate current increases. is flowing between the anode and cathode, suflicient to pick up the relay I5,.this relay will be energized and at that moment the. burner arrangement IIJ is energized .sof that the motor can pump the fuel and air out the nozzle and the ignition can be provided. At the same time the contacts 50 provide a circuit for picking up the relay 49 which in turn may provide a holding When a predetermined current circuit for itself, and at the Sametime picks up the contacts 44, so as to close the capacitor charg- Ing circuit which provides the negative bias for the control electrode 39. Thus, a predetermined time is given the furnace to obtain its predetermined normal condition of operation, that is, a normal flame condition, which time is measured from the time relays I5 and 49 close until electrode 39 becomes suciently negative so as to decrease the anode current a predetermined or sufficient amount so that the relay I5 will drop out. In order to provide the desired negative bias the voltage across thel winding 4I produces a dow of current in the biasing circuit which is rectified by the rectfying plate 40 and cathode 3|, and the capacitor 42 is charged with the polarity indicated, and at a rate depending upon the constants of the circuit..such as the voltages across the transformer windings, the value of the capacitor 42 and of the resistors 46 and 41. Since the anode current will increase to a sufficient amount to cause the relay I5 to be energized before the control electrode charging circuit is even closed, the cathode-anode circuit may reach its normal operation before the control electrode 39 obtains a negative bias. However, as the negative bias on the control electrode continues to increase the plate current will thereby begin to decrease and will continue to decrease until a sufliciently low current is obtained so that the relay I5 is allowed to drop out. When this happens the time given for the burner mechanism I0 to reach a predetermined normal state of operation will have ended. The predetermined time delay will, therefore, be determined by the constants of the circuit which charges the capacitor 42 and the characteristics of the relay I5 which determines the minimum current necessaryto maintain the relay in its closed position.

If, however, the predetermined normal condition of operation of the electrically operated de- A vice, or in other words the proper flame, has obtained before the relay I5 drops out, I have provided an arrangement responsive 'to the attainment of this condition for overcoming the negative bias in order that the electric discharge device 29 may remain conductive so that the relay I5 may remain picked up. I have, therefore, employed a dame detector according to that described in the above mentioned Patent 2,112,736, to Cockrell, which includes electrodes 56 and 51 which are placed in the path of the normal flame. The electrode 56 is connected to the transformer secondary winding 28 through a conductor 58 and the conductor 35, and other electrode 51 is connected to the control electrode 39 through conductors 59 and 48. Any other suitable type of flame detector may be employed, such as a photoelectric device, or a heat responsive device, which operates upon the application of heat when the flame is established and which may be operatively so connected to the control electrode 39 so as to allow its negative charge to leak off. Since, as the above-mentioned Cockrell patent indicates the current flows better in a direction opposite to the flame propagation, so it will be seen that the negative bias on the control electrode 39 will be removed through a circuit which includes one side of the secondary Winding 28, the conductors 35 and 58, electrodes 56 and 51, conductors 59 and 48, resistor 41, capacitor 42 and conductor 34 back to the other side of the winding 28. The capacitor 42 will be discharged at a rate and to a value depending upon several factors, such as the values of the capacitor 42, resistors 45 and 41, and the value of the voltage appearing across the secondary winding 28. If resistance 41 isA relatively small, the plate current will increase slowly when the flame is established as the control electrode is in effect connected directly to the capacitor 42 and due to the high impedance of the conduction to the flame the capacitor 42 cannot be discharged quickly. 'I'he capacitor under these conditions will probably be completely discharged or possibly charged in the reverse polarity due to the rectifying characteristics of the ame. I'he plate current hence cannot follow the fluctuations of the flame and in case of flame failure there would be a time delay before the relay I5 would drop out. If, however, resistance 41 is made a large value the control electrode is instantly made more positive when the flame is established and the plate current jumps to its full value. Hence, the capacitor 42 is discharged to a certain extent. The relative values of the circuit constants, therefore, can be adjusted so that in case of flame failure the relay I5 will drop out instantaneously or after a predetermined time delay.

In Figs. 2 to 5, inclusive, other suitable arrangements are disclosed for providing the time delay by the charging of a capacitor which provides the negative bias on the control electrode 39. In these figures the main lines, the thermostat, the oil burner arrangement, and the electric discharge device will be given the same numerals as they had in Fig. 1, while in view of the differences in the remainder of the circuits the other parts will be given new numerals. Therefore, in Fig. 2 the oil burner I0 is adapted to be connected across the lines I 6 and I1 through conductors 69, 6I, contacts 62 of a relay 63, and conductors 54, 65. The anode 38 and the cathode 3I are permanently energized from the lines I6 and I1 through an auto transformer 66, which is connected across the lines I6 and I1 through the conductors 65 and 61. Thus, the anode is connected to one side of the transformer Winding 66 through a conductor 68, the operating coil 69 of the relay 10, and a conductor 1I, while the other end of the transformer winding 66 is connected to the cathode by a conductor 12. Contacts 13 of the relay 10 are connected through an operating coil 14 of the relay 63 and conductors 64 and 65 to one side of a suitable source or the line I1, While the otherside of the con..

.tacts 13 are connected to the other side of the source, or the line I6 through a conductor 15, thermostat 22 and a conductor 16. The cathode of the discharge device 29 is kept continually heated through the winding 33 of the transformer and so a predetermined normal current flows in the plate circuit in order to keep the relay 10 permanently picked up. When the thermostat 22, thereforascalls for heat it will close its contacts and complete the circuit through the contacts 13 and the operating coil 14 of the relay 63 so as to pick up the relay 63 in order that its contacts 62 will complete the circuit of the burner operating arrangement I so that it may be energized. At the same time the thermostat 22 closes in order to energize the burner I0 through the relays and 63, contacts 11 which are operatively connected to the thermostat in any suitable manner, such as being on the thermostat and which are alsol closed in order to complete a control electrode biasing circuit. The biasing circuit as in Fig. 1, includes a' suitable capacitor 18, a source of potential in the form of a winding 19 of the transformer, resistor 80 connected in shunt with the capacitor 18 and resistor 8| which connects the negatively charged sidey of the capacitor to the control electrode 39. Thus, when the thermostat 22 calls for heat a circuit will be completed which includes the source 19, conductor 82, contacts 11, conductor 83, rectifying plate 40, cathode 3|, conductor 12, capacitor 18 and a resistor 84 back to the other side of the winding 19. The capacitor 18 will, therefore, be charged as indicated and a negative bias will be built up at a predetermined rate on the control electrode 39. When the control electrode has been biased sufficiently negative so as to reduce the anode current to a sufficient amount that the relay 10 will drop out, and the relay 63 Will also drop out and the burner mechanism will be deenergized. A suitable arrangement, however, is provided .for overcoming the bias on the control electrode 39, if the ame should obtain a predetermined normal condition of operation before the relay 10 drops out.

In order that the grid bias may be overcome if the flame obtains its normal condition within the desired predetermined time a photoelectric tube 85 is provided with its cathode 86 in line with the flame so that the light thereof may shine on the cathode and make it conductive. Any suitable type of ame detection may be employed. The cathode is, therefore, connected to the control grid through a conductor 81 which is in turn connected to the negative side of the capacitor 18 through a conductor 88 and the resistor 8|. An anode 89 of the photoelectric tube 85 is connected to the transformer 66 at any suitable point, such as its mid-point through a conductor 90. Thus, if the name 1s ignited, the photoelectric tube may be made conductive and current will flow from the mid-point of the winding 66 through the conductor 90, the photoelec- I have also illustrated in Fig. 3 a control system in which the current flowing between the anode and cathode of the discharge device 29 is normally maintained at a predetermined value, since the anode and cathode may be permanently connected to the lines I6 and I1 through a winding 9| of an autotransformer, or any other suitable arrangement. The autotransformer 9| is connected solidly to the lines I6 and I1 through the conductors 92 and 93 and one end of the winding 9| is connected to the anode 30 through conductor 94, operating coil 95 of a relay 96 and a conductor 91. vThe cathode 3| is connected to the other end of the winding 9| through the conductor 98. Thus, the relay 96 is normally picked up and the circuit for energizing the burner mechanism I0 includes conductors 99 and |00, contacts |0| of the relay 96, conductor |02, normally open contacts |03 of the relay |04, and conductors |05 and 93. Therefore, when the thermostat 22 closes its contacts upon a call for heat, relay operating coil |06 is connected across the lines I6 and |1 through the conductors 93 and |05, a conductor |01, thermostat 22` and a conductor |08. The burner mechanism |0 is, therefore, connected across the lines I6 and I1 and its operation fis, therefore, initiated. At the same time normally closed contacts |09 of the relay |04 will be opened, thus rendering eiective the circuit which produces the biasing voltage on the electrode 39. The circuit for obtaining this bias includes a capacitor ||0 which is connectedfto one side of the winding 9| through a conductor and which is also connected to the rectifying plate 40 of the discharge device 29 through a conductor II2, resistor H3, and conductor H4. The grid biasing circuit is completed through the cathode, the conductor 98 back to the other side of the capacitor ||0. The voltage'necessary to charge the capacitor ||0 is provided by the potentiometer arrangement ||5 which includes a resistor having one side connected to a tap on the Winding 9| through the conductor ||6, and the other side of the resistor being connected to an end of the winding 9| through the conductor A movable tap is provided and this is connected to the rectifying plate through a capacitor ||1, conductor ||8, and conductor H4. Thus,

the capacitor ||0 is charged with the polarity as indicated, and the negative charge is transferred to the control electrode 39 through the conductor ||2, a conductor ||9, a resistor |20, and a conductor |2|. If the ame ignites before the plate current has reduced sufciently for the relay 96 to drop out the photoelectric device 8| will be made conductive. Any other suitable name condition responsive device may, of course, be employed. The voltage necessary to overcome the grid biasing effect is provided through a potentiometer |22 which is connected to a suitable point, such as the midpoint of the Winding 9| through a conductor |23. A movable electrode thereof is connected through a resistor |24 and a conductor |25 to the anode 89 of the photoelectric device 85. Thus, the current may flow from the anode to the cathode 86 of the photoelectric device and through the conductor |2I, resistor |20, conductor ||9, conductor ||2, capacitor |'|0, conductor 98 back to the potentiometer |22l so that the charge from tiring,

on the capacitorwill be reversed and the removed.

In Fig. 4, I have illustrated a circuit in which the cathode of the electric discharge device 29 is kept in a constantly heated condition but the voltage is not impressed across the anode and cathode until the room thermostat closes. Also, in this illustrated arrangement of my invention only one relay is provided which when energized connects the burner mechanism across the lines |6 and |1 through conductors |26, |21, contacts |28 and conductor |29. An autotransformer Winding |30 is constantly energized from the lines |6 and |1 through the conductors |3| and 32. One side of the winding |30 is connected to the anode of the electric discharge device through a conductor |33, operating coil |34 of a relay |35 and conductor |36, while the other end of the winding |30 is connected to the cathode through the conductor |32, line `|1, a conductor |31, thermostat 22, and a conductor |38. Since the winding 33 which is connected to the heating arrangement for the cathode is permanently energized the cathode will be kept in a heated condition so that when the thermostat |9 is closed, current will quickly attain its predetermined normal condition so that the relay |35 will pick up in order to energize the burner mechanism I0. Closing of the thermostat 22 will also close a circuit for producing the biasing voltage on the control electrode 39. This circuit includes a capacitor |39 which is connected at one side to a tap |40 of the winding |30. The circuit may be traced from the other side of the capacitor |39 through conductors |4| and |42,

grid bias rectifying plate 40 and cathode 3| of the electric discharge device, conductor |38, thermostat 22., conductor |31, line |1, conductor |32, tap |40 to the condenser |39. The conductor |32 is also connected through conductors |43 and |44 to a capacitor |45 which is shunted by a resistor |46. This parallel circuit is in turn connected to the negative side of the condenser |39 through conductors |41 and |48, a resistor |49, and conductor 4| When the thermostat 22 closes, capacitor |39 is immediately charged and the capacitor |45 is charged as indicated at a predetermined rate in order to produce the negative bias on the control electrode 39. 'I'he negative terminal of the capacitor |45 is connected to the control electrode through the conductors |41 and |48, a resistor and a conductor |5|. If, however, the iiame is ignited before a sufficient charge is obtained on the electrode in order to stop the tube the photoelectric cell 85 will be made conducting. Here again, any other suitable iiame responsive device may be employed. A suitable voltage is provided through a potentiometer arrangement |52 which has one side connected "to a suitable point, such as the midpoint of the winding |30, through a conductor |53, and a movable element which is connected through a resistor |54 and a conductor |55 to the anode of the photoelectric -cell 8|. The potentiometer is also connected to the DOsitively charged side of the capacitor |45 through the conductor |44. Thus, if the flame is established the currentmay ow through the potentiometer, the resistor |54, con ductor |55,.photoelectric cell 8|, conductor |5|, resistor |50, the parallel circuit including the capacitor |45, and'resistor |46, the conductor |44 back to the potentiometer |52 in order to discharge the capacitor |45, so as to remove the grid bias. The resistor the capacitor |46 is put in parallel with 45, since the thermostat is in the cathode circuit and when it is open the capacitor may discharge through this parallel circuit. When the thermostat closes, the capacitor |39 is charged immediately through the tube and this voltage is supplied through the resistor |49 to the capacitor |45, so that it may be seen that the value of the resistor |49 determines the charging rate and, therefore, the time delay of the circuit.

I have illustrated in Fig. 5 another arrangement for obtaining the time delay with a suitable llame detector, such as a photoelectric tube, and the circuit is so connected that the photoelectric cell may be checked after each cycle of operation in order to insure that it is in satisfactory condition. The control electrode of the electric discharge device is connected to a negative potential whenever the thermostat is opened, and if the photoelectric tube is intact the grid will be biased negatively so as to cause the relay which is operated by the plate circuit to drop out. Then with another relay which can only be operated if the former is closed, I am able to prevent operation of the circuits if the photoelectric tube is defective. This provides an additional arrangement for making my control circuit fail-proof. It is also apparent that in all the modications shown if the electric discharge device fails the burner may not be energized. Thus, the burner will operate only when it is under proper control.

The burner means I0, as in the other gures, is adapted to be connected across the lines I6 and I1 through conductors |56 and |51, normally open contacts |58 of the relay |59 and conductor |60. A suicient voltage may be permanently impressed across the anode and cathode of the electric discharge device 29 through a transformer secondary |6| of a transformer |62 since its primary winding |63 is permanently connected across the lines I6 and |1 through conductors |64 and |65. As will be described in more detail, however, as the description proceeds, the tube will be rendered nonconductive since the grid is maintained sufficiently negative to prevent the tube from ring when a relay |66 is deenergized.

The anode the cathode 3| is connected to the other side of the winding |6| through conductors |1| and |12. An operating winding |13 of the relay |66 is connected to a secondary winding |14 of the transformer |62. This circuit may be traced from one side of the winding through conductors |15,

operating winding |13, conductor |11, the normally closed contacts |18 of the relay |69, conductors |19 and |80, thermostat 22 back to the other side of the winding |14 through a conductor |8|. When the thermostat 22 calls for heat and closes its contacts, the relay |66 will be picked up through the circuit which has just been traced. 'This will close contacts |82 and |83 As will be described below,

rent to flow through the anode and cathode circuit in order to energize the relay |69. When the relay |69 picks up it closes its contacts |84 and since contacts |82 of the relay |66 have been closed, the relay |59 will be energized through a circuit which includes the conductor |15, operating coil |85 of the relay |59, contacts |82 of the relay |66, conductor |86 to the condropping out as its tacts |84, conductor |19, conductor |80, thermostat 22 back to the other side of the transformer secondary |14 through the conductor |8|. Also, when the relay |69 picks up it opens its contacts |18. However, relay |66 is prevented from coil remains energized through a circuit which includes conductor |15,

,operating coil |13 of relay |66, a resistor |81,

conductor |80, thermostat 22, and back to the other side of transformer secondary |14 through the conductor |8|. The energization of the relay |59 will cause the burner mechanism to be energized. Furthermore, the closing of the contacts |83 of the relay |66 will cause the control electrode biasing circuit to be energized so that a negative bias may be provided at a predetermined r'ate on the control electrode 39. The grid bias is provided by charging a capacitor |88 which is s'hunted with a resistor |89 for the same reason thatthe capacitor in Fig.\ 4 was shunted by a resistor. -One side of this parallel resistor-capacitor combination is connected to one side of the transformer secondary |6| through conductors |90 and |12, and the other side of this parallel circuit is` connected through a conductor |9| to the contacts |83 of the relay |66. The circuit may then be traced through conductors |92 and |93, resistor |94, conductor |95, capacitor |96, a conductor |91 back to a tap |98 on the secondary Winding I6I. The circuit for passing current through the"` photoelectric tube 85 when it becomes energized includes a potentiometer |99 which is connected to any suitable tap 200 of the winding |62 through a conductor 20|, a resistor 202, a conductor 203, photoelectric cell 85, a resistor 204, conductor |92, contacts |83, conductor I9I, capacitor |88, conductor |12, back to the potentiometer |99.

When the thermostat 22 in Fig. 5 closes, the

Y relay |66 will 'oe energized which in turn closes the contacts I 83 so that the negative bias, which is present when the relay |66 is dropped out, is removed from the control electrode 39 through a conductor 205, resistor 204, conductor |92, contacts |83, conductor I9I, capacitor |88 whose voltage at this instant is zero, and then to cathode 3| through the conductor |1I, thus allowing the relay |69 to pick up. This in turn provides a circuit for energizing the relay |59 in order to energize the burner mechanism, At the same time due to the closing of the contacts |33, a circuit will be established for providing the voltage bias to be produced on the control electrode 39 at a predetermined rate. This circuit for providing the grid bias may be traced first through the tap |98, capacitor |96, conductor |95, through the rectifying plate 40 and cathode 3| of the electric discharge device 29 back to the tap |98 through the conductors |1| and |12 and a portion of the winding |6|. This immediately provides a flow of current from capacitor |96, tap |98, a portion of the secondary winding |6|, conductors |12 and |90, the capacitor and resistor combination |89 and |89, conductor |9I, contacts |83, conductors |92 and |93, resistor |94, conductor |95 back to the capacitor |96. The capacitor |88 will be charged as indicated and it will be seen that the negative side of the capacitor will be connected to the control electrode 39 through the conductor I9 I, contacts |83, conductor |92, the resistor 204, and a conductor 205. If the flame is not established before a suicient bias is provided on the control electrode 39 the anode current will become suciently small so that the relay |69 will drop out and the relay |59 will also drop out and cause the burner mechanism I0 to be deenergized. However, if the ame has reached its predetermined condition the photoelectric cell will become conductive and the voltage impressed across the potentiometer |99 will cause a current to flow through the resistor 202, conductor 203 to the photoelectric cell anode and cathode, conductor 205, resistor 204, conductor |92, contacts |83, conductor |9|, through the capacitor |88 in a direction opposite to which it has been charged, back to the potentiometer |99 through the conductors |90 and |12. This` will reverse the charge on the capacitor, and remove or overcome the grid bias. The burner mechanism may, therefore, run until the call for heat in the room to be heated has been satisfied and the thermostat 22 moves its contacts to an open position.

When the thermostat opens its contacts the relay |66 will be deenergized so that its contacts |83 will be open circuited and the tube to be made .non-conducting so that the relay |69 will also drop out. 'I'he tube is made non-conductive, since when the contacts |83 open, the control electrode 39 will be connected through the conductor 205, resistor 204, conductor |93, resistor |94, conductor |95, to the negative side of the capacitor |96. Since the capacitor |96 is kept constantly charged through the circuit which includes the rectifying plate and cathode, the grid will be maintained negatively during the time when the contacts |83 are open so as to prevent firing of the tube during that time. As has already been mentioned, this circuit also allows the flame responsive device to be checked, since when the flame goes out, if the photoelectric tube continues to pass current it is defective, and the negative charge on the grid will be overcome through the circuit which has already been described. This will allow the electric discharge device to continue to pas's a suicient current to keep the relay |69 energized and since the other two relays have already been dropped out due to the opening of the thermostat 22, it will be impossible to energize these relays again so long as the relay |69 is picked up, so that if the thermostat were again to call for heat the burner mechanism would not be energized. This is because of the resistor |81, which is of suitable magnitude, has been shunted across the contacts |18. The resistance of the resistor |81 is, therefore, so set that current owing through it is limited to such a value belowJ that required'to energize the relay |66. Therefore, the circuit'to energize the relay |66 must include the contacts |18 of relay |69. However, after relay |66 has been energized a sufficient current will pass through the resistor |81 to hold the relay |66 in closed position. Therefore, if the relay |66 has dropped out and the relay |69 is still picked up the contacts |18 will be open and the current passing through the circuit including the resistor |81 and the coil |13 is not suiicient to pick up the relay |66. Thus, closing of the thermostat 22 will not cause the burner mechanism to be energized under these conditions. If, however, the photoelectric cell is working properly, when the thermostat 22 is open, current will not pass through the cell so as to overcome the grid bias and the tube will cease firing and the relay |69 will drop out, thus providing a circuit through contacts |18 so that relay |66 may befenergized Whenever the thermostat 22 again calls for heat. It will also be apparent that any other suitable condition responexample of an application to which my improved control system iinds an ecient use, I have described it in its application to a control circuit for an oil burner. However, it is to be understood that my control system may be employed with any electric device whose operation it is desired to be stopped provided a normal operation is not obtained within a predetermined time. It is also to be noted that although my improved control circuit in the illustrated embodiments has been described in connection with a continuous ignition system for an oil burner circuit, it is to be understood that my improved system may be equally well employed to' an ignition circuit of the intermittent type. I should also like it to be understood that any other suitable rectifying arrangement other than the rectifier plate and cathode in the electric discharge tube may be employed for rectifying the current which charges the capacitor, the negative terminal of which is connected to the control electrode. Thus, a separate rectifying tube may be employed in series with an alternating source of supply or a direct current source of supply could be employed to charge the capacitor.

Modifications of the particular arrangements which I have disclosed embodying my invention will occur to those skilled in the art, so that I do not desire my invention to be limited to the particular arrangements set forth and I intend in the appended claims to cover all modications which do not depart from the spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent in the United States is:

l. A control system for an electrically operated device including an electric discharge device having an anode, a cathode and a control electrode, a condition responsive device, means including said condition responsive device and said electric discharge device for initiating operation of said electrically operated device, means for producing a unidirectional biasing voltage on said control electrode at a predetermined rate so that after a predetermined time delay a lowering of current flowing through said discharge device to a predetermined value will cause said electrically operated device to be deenergized, means for preventing actuation of said control electrode potential changing means until operation of said electrically operated device is initiated, and means responsive to a predetermined normal condition of operation of said electrically operated device for overcoming said biasing voltage s that said electrically operated device will remain energized when said predetermined normal condition of operation obtains before the expiration of said predetermined time delay.

2. A control system for an electrically operated condition changing device including an electric discharge device having an anode a cathode and a control member,l a condition responsive device, means including said condition responsive device and said electric discharge device for initiating operation of said condition changing device, a biasing circuit including a capacitor interconnected between said cathode and said control electrode for producing a change in an electrical condition on said control member, an independent circuit starting means for charging said capacitor at a predetermined rate so that after a predetermined time delay a lowering of the current flowing through said discharge device to a predetermined value will cause said condition changing device to be deenergized, means for closing said charging circuit when operation of said condition changing device is initiated, and means responsive to a predetermined normal condition of operation of said condition changing device and connected with said biasing circuit for overcoming said change in an electrical condition on said control member so that said condition changing device will remain energized when said predetermined normal condition of operation obtains before the expiration of said predetermined time delay.

3. A control system for a furnace having electric means for supplying combustibles and ignition thereto including an electric discharge device having an anode a cathode and a control electrode, a thermostat, means including said thermostat and said electric discharge device for initiating operation of said electric means, a biasing circuit including a capacitor for producing a biasing voltage on said control electrode, a charging circuit including means for charging said capacitor at a predetermined rate so that after a predetermined time delay a lowering of current owing through said discharge device to a predetermined value will cause said electrically operated device to be deenergized, means for initiating actuation of said capacitor charging means when operation of said electric means is initiated, and means responsive to a proper ame condition in said furnace and connected with said biasing circuit for overcoming said biasing voltage so that said electric means will remain energized when said proper flame condition obtains before the expiration of said predetermined time delay.

4. A control system for an electrically operated device including a source of potential, an electric discharge device having an anode a cathode and a control electrode, a condition responsive device, means including said condition responsive device and said electric discharge device forrinitiating operation of said electrically operated device, an electrical energy storage device, means for operatively connecting said energy storage device to said source so as to charge said energy storage device at a predetermined rate, said operatively connecting means for actuating said connecting means when said electrically operated device is initiated, separate means for connecting the negatively charged side of said energy storage device to supply a unidirectional biasing voltage to said control electrode so that after a predetermined time delay a lowering of current flowing through said discharge device to a predetermined value will cause said electrically operated device to be deenergized, and means responsive to a predetermined normal condition of operation of said electrically operated device for overcoming said biasing voltage so that said electrically operated device will remain energized when said predetermined normal condition of operation obtains before the expiration of said predetermined time delay.

5. A control system for an electrically operated device including a source of alternating potential, an electric discharge device having an anode and a cathode, a condition responsive device, means including said condition responsive device and said electric discharge device for initiating operation of said electrically operated device, a control electrode and a rectifying plate within said electric discharge device,l a biasing circuit including a capacitor interconnected betweensaid cathode and said controlelectrode, a

Aseparate charging circuit including said rectifying plate and said cathode for charging said capacitor to produce a biasing voltage on said control electrode at a predetermined rate so that after a predetermined time delay a lowering of current ilowing through said electric discharge device to a predetermined value will cause said electrically operated device to be deenergized, means for closing said charging circuit when the operation of said electrically operated device is initiated, and means responsive to a predetermined normal condition of operation of said electrically operated device for overcoming said biasing voltage so that said electrically operated device will remain energized when said predetermined normal condition of operation obtains before the expiration of said predetermined time delay.

6. A control system for an electrically operated device including an electric discharge device having a control member, a biasing capacitor for said control member, rectifier means for charging said capacitor, switch means including a main switch and an auxiliary switch, means including said main switch means and said discharge device ior initiating operation of said electrically operated device upon a predetermined current flowing through said discharge device, means including` -said auxiliary switch for interconnecting said rectifying means and said capacitor to produce a biasing voltage on said control member at a predetermined rate so that after a predetermined time delay a lowering of current flowing through said discharge device to a predetermined value will cause said electrically operated device to be deenergized, operation of said biasing voltage means being initiated when the operation of said electrically operated device is initiated, and means responsive to a predetermined normal condition of operation of said electrically operated device for overcoming said biasing voltage so that said electrically operated device will remain energized when said predetermined normal condition of operation obtains before the expiration of said predetermined time delay. Y

7. A control system for a furnace having electric means for supplying combustibles and ignition thereto including an electric discharge device having a control member, a biasing capacitor for said member, means for charging said capacitor, a thermostat within a space to be heated by said furnace, means including said thermostat and said discharge device for initiating operation of said electric means upon a predetermined current ilowing through said discharge device, `means responsive to said predetermined current flow through said discharge device for interconnecting said rectifying means with said condenser to produce a biasing voltage on said control member at a predetermined rate so that after a predetermined time delay a lowering of current owing through said discharge device to a predetermined value will cause said electric means to be deenergized, and means responsive to a proper flame in said furnace for overcoming said biasing voltage so that said electric means will remain energized when said proper flame obtains before the expiration of said predetermined time delay.

8. A control system for a furnace having electric means for supplying combustibles and ignition thereto including an electric discharge device having an anode a cathode a control electrode and a separate rectifying plate, a temperature responsive device, means including said temperature responsive device and said discharge device for initiating `operation of said electric means upon a predetermined current flowing between said anode and cathode, a source of alternating potential, a biasing circuit including an energy storage device connected between said cathode and said control electrode, a charging circuit including said source energy storage means cathode and rectifying plate for charging said energy storage device to produce a unidirectional a biasing voltage on said control electrode at a predetermined rate so that after a predetermined time delay the current owing between said anode and cathode will have been reduced to a predetermined value so that said electric means will be deenergized, means responsive to said predetermined current for rendering operative said charging circuit when operation of said electric means is initiated, and means responsive to a proper llame in said furnacefor overcoming said biasing voltage so that said electric means will remain energized when said proper llame obtains before the expiration of said predetermined time delay.

9. A control system for an electrically operated device including a source of alternating potential, an electric discharge device having an anode a cathode and a control electrode, means normally energized from said source for maintaining said cathode in a constantly heated condition, means including a condition responsive device adapted to call for operation of said electrically operated device for connecting said anode and cathode across said source of potential, means responsive to a predetermined flow of current through said discharge device for initiating operation of said electrically operated device, means rendered effective upon connection of said cathode to said source for producing a unidirectional biasing -voltage on said control electrode at a predetermined rate so that after a predetermined time delay a lowering of the current flowing through said discharge device to a predetermined value will cause said electrically and means responsive to a predetermined normal condition of operation of said electrically operated device for overcoming said biasing voltage so that said electrically operated device will remain energized when said predetermined normal condition of operation obtains before the expiration of said predetermined time delay.

10. A control system for an electrically operated condition changing device including a source of alternating potential, an electric discharge device having an anode a cathode and a control member, means normally energized from said source for maintaining said cathode in a constantly heated condition, means including a thermal responsive device adapted to call for operation of said condition changing device for connecting said anode and cathode across said source of potential, means responsive to a predetermined flow of current through said discharge device for initiating operation of said condition changing device, means including said current responsive means for producing a unidirectional change in an electrical condition on said control member at a predetermined rate so that after a predetermined time delay a lowering of the current flowing through said discharge device to a predetermined value will cause said condition changing device to be deenergized, and means responsive to a predetermined normal condition of operation of said condition changing device for overcoming said change in said electrical condition of said control member so that said condition changing device Will remain energized when said predetermined normal condition of operation appears before the expiration of said predetermined time delay.

1l. A control system for an electrically operated device including a source of alternating potential, an electric discharge device having an anode a cathode and a control electrode, meansincluding a condition responsive ldevice adapted to call for operation of said electrically operated device for connecting said anode a-nd cathode across said source of potential, means including a relay for connecting said electrically operated device across said source, an operating coil for said relay, means for connecting said coil in series with said anode and cathode so that said relay will be energized when the current flowing through said electric discharge device is a predetermined value,. means under the control of said relay for producing a unidirectional biasing voltage on said control electrode at a predetermined rate so that a lowering of the current flowing through said discharge device to a predetermined value will allow said relay todrop out and deenergize said electrically operated device, means responsive to energization of said relay for energizing said biasing voltage means, and means responsive to a predetermined normal condition of operation of said electrically operated device for overcoming said biasing voltage so that said electrically operated device will remain energized when said predetermined normal condition of operation obtains before the expiration of said predetermined time delay.

12. A control system for a furnace having electric means for supplying combustibles and ignition thereto including, a source of alternating potential, an electric discharge device having an anode a cathode and a control electrode, means including a thermostat within the space to be heated by the furnace for connecting said anode and cathode across said source of potential,

means responsive to a predetermined current flowing between said anode and cathode for initiating operation of said electric means, a rectifying plate within said discharge device, an energy storage device, a transformer adapted to be energized from said source, a circuit closed under the control of said current responsive means and including said transformer energy storage device cathode and rectifying plate for producing a unidirectional biasing voltage on said control electrode at a predetermined rate so that after a predetermined time delay a lowering of the current flowing between said anode and cathode will cause said electric means to be deenergized, and means responsive to a proper flame condition in said furnace for overcoming said biasing voltage so that said electric means will remain energized if said proper flame obtains before the expiration of said predetermined time delay.

13, A control svstem for an electrically operated device including an electric discharge device having an anode a cathode and a control electrode, means for maintaining said cathode in a constantly heated condition, means for constantly impressing a voltage across said anode and cath- Cil ode, means including a condition responsive device for controlling said electric discharge device to effect current flow therethrough, a relay responsive to the current flow through said discharge device for initiating operation of said electrically operated device, means controlled by said condition responsive device for producing a biasing voltage on said control electrode at a predetermined rate so that after a predetermined time delay a lowering of current flowing through said discharge device to a predetermined value will cause said electrically operated device to be deenergized, and`means responsive to a predetermined normal condition of operation of said electrically operated device for overcoming said biasing voltage so that said electrically operated device Will remain energized when said predeter` mined normal condition of operation obtains before the expiration of said predetermined time delay.

14. A control system for a furnace having electric means for supplying combustibles and ignition thereto including an electric discharge device having an anode a cathode and a control electrode, means for maintaining said cathode in a constantly heated condition, means for constantly impressing a voltage across said anode and cath- 0de to effect current flow therebetween, means including a thermostat and a relay responsive to said current flow through said electric discharge device for initiating operation of said electric means, means including a second relay under the joint control of said thermostat and said first relay for producing a biasing voltage on said control electrode at a predetermined rate so that after a predetermined time delay a lowering of current owing through said discharge device to a predetermined value will cause said electrically operated device to be deenergized, means responsive to a proper flame condition in said furnace for overcoming said biasing voltage so that said electric means will remain energized when said proper ame condition obtains before the expiration of said predetermined time delay.

15. A control system for an electrically operated device including a source of alternating potential, an electric discharge device having an anode a cathode and a control electrode, means including a transformer for maintaining said cathode in a constantly heated condition and for connecting said source across said anode and cathode so as to produce a predetermined current therebetween, a relay having an operating coil in said anode circuit and having contacts in a circuit including said electrically operated device and said source, means including a condition responsive device and a second relay under the joint control of said first relay and said condition responsive device and a third relay under the joint control of said first and second relays for completing said electrically operated device circuit, means under the control of said second relay for producing a biasing voltage on said control electrode at a predetermined rate so that after a predetermined time delay a. lowering of current flowing between said anode and cathode to a predetermined value will cause said electrically operated device to be deenergized, and means responsive to a predetermined normal condition of operation of said electrically operated device for overcoming said biasing voltage so that said electrically operated device will remain energized when said predetermined normal condition of operation obtains before the expiration of said predetermined time delay.

16. A control system for a furnace having electric means for supplying combustibles and ignition thereto including an electric discharge device having an anode a cathode a control electrode and a rectifying plate, a relay having an operating coil in series with said anode and contacts in a circuit including said electric means, means for permanently connecting said source across said anode and cathode producing a predetermined normal current therebetween, means including a thermostat within a space to be heated for completing the circuitV including said relay contacts and electric means when said thermostat calls for heat, an energy storage device, a circuit including said energy storage device, cathode, and rectifying plate for producing a biasing voltage on said control electrode at a predetermined rate so that after a predetermined time delay the current flowing between said cathode and anode will have been reduced a suiiicient amount so that said relay will be deenergized and said electric means turned off, means including switch means responsive to movement of said thermostat to said heat calling forrposition for connecting said biasing circuit to said source, and means responsive to a proper ame condition in said furnace for overcoming said biasing voltage so that said electric means will remain energized when said flame obtains before the expiration of said predetermined time delay.

17. A control system for a furnace having electric means for supplying combustibles and ignition thereto including an electric discharge device having an anode a cathode a control electrode and a rectifying plate, a relay having an operating coil in series with said anode and contacts in a circuit including said electric means, means including a transformerfor'permanently connecting said source across said anode and cathode so as to produce a predetermined normal current; therebetween, means including a thermostat Within a space to be heated for completing said circuit including said relay contacts and said electric means when said thermostat calls for heat, a potentiometer connected across a portion of said transformer, a capacitor, a circuit including said potentiometer, capacitor, rectifying plate and cathode for providing a biasing voltage on said control electrode at a predetermined rate so that after a predetermined time delay the current flowing between said anode and cathode will have been reduced a suicient amount so that said relay will be deenergized and said electric means turned off, normally closed contacts for short circuiting said capacitor, means for opening said contacts for initiating the biasing of said control electrode when said thermostat calls for heat, and means responsive to a proper iiame condition in said furnace for overcoming said biasing voltage so that said electric means will remain energized when said proper flame condition obtains before the expiration of said predetermined time delay.

18. A control system for a furnace having electric means for supplying combustibles and ignition thereto including a source of potential, an electric discharge device having an anode a cathode and a control electrode, a relay having an operating coil in series with said anode and contacts adapted to close when said relay is energized, means including a transformer for connecting said source across said anode and cathode so as to produce a predetermined current therebetween, a thermostat within the space to be heated, Va second relay having an operating coil in series with said thermostat and contacts adapted to close when said second relay is energized in series with said contacts of said first mentioned relay, means including said contacts of said first and second mentioned relays for connecting said electric means across said source when the thermostat calls for heat, means for connecting said control electrode to a source of negative potential when said second mentioned relay is deenergized, and contacts on said first mentioned relay adapted to be closed when said rst mentioned relay is deenergized in series with said operating coil of said second mentioned relay and said thermostat so that said first mentioned relay must move to its deenergized position before said second mentioned relay can be energized when said thermostat moves to its circuit closing position upon a callfor heat.

19. A control system for a furnace having electric means for supplying combustibles and ignition thereto including a source of potential, an electric discharge device having an anode a cathode and a control electrode, a relay having an operating coil in series with said anode and contacts adapted to close when said relay is energized, means including a transformer for connecting said source across said anode and cathode so as to produce a predetermined current therebetween, a thermostat within the space to' be heated, a second relay having an operating coil in series with said thermostat and having contacts adapted to close when said second relay is energized in series with said contacts of said first mentioned relay, means including said contacts of said first and second mentioned relays for connecting said electric means across said source when the thermostat calls for heat, means for producing a biasing voltage on said control electrode at a predetermined rate so that after a predetermined time delay a lowering of current flowing through said discharge device to a predetermined value will cause said electrically operated device to be deenergized, means responsive to a predetermined normal condition for operation of said electrically operated device for overcoming said biasing means so that said electrically operated device will remain energized when said predetermined normal condition of operation obtains before the expiration of said predetermined time delay, and contacts on said first mentioned relay adapted to be closed when said first mentioned relay is deenergized in series with said operating coil of said second mentioned relay and said thermostat so that said first mentioned relay must move to its deenergized position before said second mentioned relay can be energized when said thermostat moves to its circuit closing position upon a call for heat.

20. A control system for a furnace having electric means for supplying combustibles and ignition thereto including a source of potential, an electric discharge device having an anode a cathode and a control electrode, a relay having an operating coil in series with said anode and contacts and adapted to close when said relay is energized, means including a transformer for connecting said source across said anode and cathode so as to produce a predetermined current therebetween, a thermostat within the space to be heated, a second relay having an operating coil in series with said thermostat and contacts adapted to close when said second relay is energized in series with said contacts of said first mentioned relay, means including' said contacts of said first and second mentioned relays for connecting said electric means across said source when said thermostat calls for heat, a capacitor, a. circuit connected to said transformer and including said capacitor for producing a biasing voltage on said control electrode at a predetermined rate so that after a predetermined time delay a lowering of current owing through said discharge device will cause said rst and second mentioned relay to drop out and said electric means to be deenergized, switch means operable to a closed position in response to movement of said second relay to a closed position for completing said control electrode biasing circuit, contacts on said first mentioned relay in series with said operating coil of said second mentioned relay so that said second mentioned relay cannot be energized unless said rst mentioned relay is deenergized, and means responsive to a proper flame condition in said furnace for overcoming said biasing voltage so that said electric means will remain energized if said ame obtains before the expiration of said predetermined time delay, and means to connect said control electrode with said cathode so that said discharge device will be rendered non-conductive when said thermostat moves to open circuit position unless said ilame responsive means overcomes the biasing voltage on said control electrode.

21. A control system for an electrically operated device including a source of alternating potential, an electric discharge device having an anode and a cathode, means normally energized maintaining said cathode in from said source for a constantly heated condition, means for constantly connecting said source across said anode and cathode to eilect current flow therebetween,

a relay responsive to said current flow, a condition responsive device, a second relay having an energizing circuit under the joint control of said condition responsive device and said rst relay and having a holding circuit under the control of said condition responsive device, a third relay under the joint control of said first and second relays for initiating operation of said electrically operated device, a control electrode and rectifying plate within said electric discharge device, a biasing circuit including a capacitor interconnected between said control electrode and said cathode, a charging circuit under the control of said second relay and including said rectiiying plate and said cathode for charging said capacltor to produce a unidirectional voltage on said control electrode at a predetermined rate so that after a predetermined time delay a lowering of current flowing through said electric discharge device to a predetermined value will cause said electrically operated device to be deenergized, and means responsive to a predetermined normal condition of operation of said electrically operated device for overcoming said biasing voltage so that said electrically operated device will remain energized if said predetermined normal condition of operation obtains before the expiration of said predetermined time delay.

ELBERT D. SCHNEIDER.

CERTIFICATE oF CORRECTION. Patent No. 2, 299,501. october 2o, 19kg.

ELBEBT'D. SCHNEIDER.

It is hereby certifie@ that error appears in the printed specification of the above numbered patent requiring correctionas follows: Page 8, seo- .ond column, line 5l, claim 9, after "electrically" insert "operated device to be deenergized,; page A9, second column, line 59, claim lh., before "means" insert -and; and that the said. Letters Patent should bev raad with this correction therein that the same may conform to therecord of thecase in /the Patent offi ce Signed and sealed. this 8th day of December, A. D. 1911.2.

A Henry Van Arsdale, (Seal) Acting Commissioner of Patents.

CERTIFICATE CF CORRECTION. Patent No. 2 299,5o1. 1 october 2o, 19kg.,

ELBERT D. SCHNEIDER.

It is'hereby certified that error appears in the printed specification of the above numbered patent requiring correctionas followsgvPage 8, sec- .ond coluRIn, line 51, claim 9, after "electrically" insert operted device to be deenergizedQ-q page 9, second column, line 59, claim 11|., before means insert -and; and that the said'Letters Patent should Deread with this correction therein that the seme may conform to therecord of thelcase in fthe Patent office.

Signed and sealed this thiday of December, A. D. 1914.2.

- Henry Van Arsdale, A(Seel) Acting Commissioner of'Patents. 

